The 21in big booster THAAD has simply never ever had any funding allocated by the US government, and as far as I can tell it has never been more then a notional upgrade made possible by the end of the ABM treaty studied a bit with company money. The idea is at least as old as the Bush pullout from the ABM treaty, but I wouldn't be surprised if it was considered of even earlier.

Since land based SM-3 is now being pursued anyway no real reason exists to invest in a more capable THAAD, compared to just placing SM-3 on the same launchers as THAAD at some future point. Both were designed on the basis of Mk41 compatibility after all and are very similar sizes. Before land SM-3 was an option, KEI also made a more capable THAAD redundant though to a lesser degree since KEI was going to be so big and expensive.

THAAD is roughly the same length but significantly smaller in diameter and weight. In fact with a booster on the bottom of the stack it wouldn't fit in a Mk41 cell.

Like I said, it was to be 21in, this was to be a complete booster replacement with no added length. That fits fine. I poked around a bit, and in fact it seems this idea goes all the way back to the early 1990s when THAAD was still joint Army-Navy in the first place. It is entirely compatible with Mk41. Since SM-3 Block II already replaces its own 13.5in booster with a 21in booster the project currently has very little justifiable point. THAAD has an added endo capability that SM-3 does not, but a massively increased motor size doesn't really do anything to exploit that. A bigger booster would also drive up the cost per missile considerably, as it does for SM-3 which is the last thing anyone wants.

Makes sense -- without MKV to allow for a smaller terminal kill stage and thus higher delta-V to increase the footprint of SM-3...it's a very constrained system limited by the need to fit within a Mk41 VLS cell using current solid propellants.

Makes sense -- without MKV to allow for a smaller terminal kill stage and thus higher delta-V to increase the footprint of SM-3...it's a very constrained system limited by the need to fit within a Mk41 VLS cell using current solid propellants.

And there aren't enough of the bigger, Mk57 cells, to justify a larger missile.

Actually, you didn't say it was a complete booster replacement, and I thought I read somewhere that the "booster" was just adding the one from SM-3 Block I to THAAD, increasing it's length.

Well I would have said third stage if I meant that, and SM-3 Block I is not 21in.

As for missile size, actually that problem was studied in the early 2000s when the US was still uncertain on building KEI owing to its immense size and absurd estimated cost, up to 70 million per warshot. The weapon was called ‘six shooter’ and ‘Standard Missile 27’ in notional studies. What they would have done is take the space of an eight cell Mk41 VLS unit and replaced it with six staggered 27in diameter cells which also extended out of the deck somewhat, I never saw a spec for just how far. A missile capable of 1,500km range and velocity as high as 6.5km/s was estimated to be possible with a 50kg kill vehicle. This weapon would be marginally capable of boost phased intercepts of ICBMs. A weapon like this is still an option and a modification reasonable for existing warships, the problem remains cost. SM-3 Block II is estimated at around 24 million dollars right now, against about 10 million each for SM-3 Block I and THAAD. A 27in missile will cost even more unless very large numbers were produced.

Wouldn't it be easier and less expensive to just add SM-3's to the THAAD battery? I believe the SM-3 can be cued by the TPY-2 radar and you can develop a common canister. That would give you a 3-stage missile with a KV for the exoatmospheric segment of the mission. Think it would take a lot less money than trying to give the THAAD missile SM-3 performance.